Digital Signal Processing (DSP) techniques enable to compensate the impairments affecting optical signals during transmission over a medium such as optical fiber by applying the inverse filter properties of the impairments. This enables higher capacity transmission over an optical fiber or transmission on longer reach of an optical fiber. These techniques can be applied to the received signals inside a receiver and a DSP used to demodulate it.
However, the application of the digital signal processing techniques is not limited to the receiver side; the techniques can also be applied to the transmitter side for additional benefits. In this configuration, DSP is used with Digital to Analog Converters (DACs), which convert the processed digital signals into analog signals with which an optical carrier is modulated. In the following description, we call a digital transmitter a transmitter equipped with DSP and DAC to emit an optical signal according to the information of processed digital signals. In such a manner, DSP of a digital transmitter can be used to pre-compensate at the transmitter side for imperfections of the transmitter hardware and to improve system performance. Digital transmitters also enable the pre-compensation of linear impairments of transmission in the fiber such as chromatic dispersion (CD) and of nonlinear impairments. Depending on source, such compensation can be called pre-compensation, pre-equalization, or pre-distortion.
An example of a transmitter equipped with DSP and DAC is described in patent literature 1 (PTL1). The optical transmission device described in PTL1 has a digital signal processing unit, a D/A converter, a polarization-multiplexing modulation unit, a light-receiving unit, a detect unit, and a correct unit.
The digital signal processing unit acts as a drive signal generate unit for generating a drive signal. The polarization-multiplexing modulation unit modulates an optical signal in accordance with the drive signal. The detect unit detects a fluctuation of a signal component of the drive signal with respect to an optical signal output by the modulation unit. The correct unit corrects a parameter of the drive signal generate unit in accordance with a detect result of the detect unit so that a non-linear characteristic of the modulation unit gets closer to a linear characteristic.
It is said that a non-linear characteristic of the modulator can be compensated according to the optical transmission device described in PTL1.